Ribonucleotide reductase (RR) has an essential role in DNA synthesis and repair and is a therapeutic target in a number of different cancers. Previous studies have shown that RNAi-mediated knockdown of either the RRM1 or RRM2 subunit sensitizes cells to the cytotoxic effects of the nucleoside analogs and more recently it has been shown that RRM2 knockdown itself has a growth inhibitory effect. Here we compare the effects of siRNA-mediated knockdown of both RRM1 and RRM2 subunits of RR in A549 and HCT-116 cells using an optimized transfection protocol. Growth of A549 cells was strongly inhibited by efficient siRNA-mediated silencing of either RRM1 or RRM2, and knockdown of each subunit led to long-term growth inhibition and cell-cycle arrest. Knockdown with sub growth inhibitory siRNA concentrations sensitized A549 and HCT-116 cells to gemcitabine when RRM1 was targeted, whereas RRM2 knockdown led to hydroxyurea sensitization. These results suggest that the inhibition of cell growth, rather than drug sensitization, is the major effect of RRM1 and RRM2 knockdown. In an A549 xenograft model, cells transfected with RRM1-specific siRNA failed to form tumors in 6 out of 8 CD1 nude mice, whereas those transfected with RRM2-specific siRNA grew but at a reduced rate. Taken together, these data demonstrate that siRNA-mediated knockdown of the RRM1 subunit is more effective than knockdown of RRM2 in inhibiting the growth of cancer cell lines and suggest that RRM1 is a potential target for nucleic acid-based cancer therapies, either alone or in combination with gemcitabine.